1. Field of the Invention
The present invention relates to a gear ratio control method for a continuously variable transmission in a vehicle, in which a starting clutch is operable to automatically engage and disengage according to a rotary speed of an engine, and a continuously variable transmission unit is operable to continuous speed change according to the rotary speed of an input shaft; and in which, the starting clutch and the continuously variable transmission unit are connected in series between a crankshaft of the engine and an output shaft. More particularly, the present invention relates to a gear ratio control method for a continuously variable transmission in which a target engine rotary speed is computed based on a vehicle speed and a throttle opening, and the gear ratio is controlled based on such computed target engine rotary speed.
2. Description of the Background Art
There are several known power units which include an engine and a hydrostatic continuously variable transmission including a swash-plate-type hydraulic pump and a swash-plate-type hydraulic motor, and in which the hydraulic pump is driven to rotate by the engine, a swash plate angle in at least one of the hydraulic pump and the hydraulic motor is controlled to change an input rotary speed of the hydraulic pump, and an output is taken out as an output rotary speed of the hydraulic motor. An example of such known power unit is disclosed in the Japanese Patent Laid-open No. 2005-263143.
Further, another example of such known power unit is disclosed in the Japanese Patent Laid-open No. 2006-200727 (particularly FIGS. 1 and 9 thereof). The Japanese Patent Laid-open No. 2006-200727 discloses not only a power unit including a hydrostatic continuously variable transmission but also a power unit including a belt-type continuously variable transmission having a metallic belt wrapped around a drive pulley and a driven pulley.
In the power unit disclosed respectively in the Japanese Patent Laid-open No. 2005-263143 and the Japanese Patent Laid-open No. 2006-200727, since a starting clutch is disposed on the output shaft side, torque control can be carried out at a position which is the nearest to an axle, and smoothness of vehicle starting is secured. In addition, it is possible to achieve an appropriate creep, thereby facilitating the driving-around of the vehicle at the time of a vehicle starting on a slope, at the time of garaging a vehicle, and so on.
At the time of starting of a vehicle (motorcycle or four-wheel vehicle) equipped with the power unit, such as disclosed in the Japanese Patent Laid-open No. 2005-263143 and the Japanese Patent Laid-open No. 2006-200727, an operation of the throttle in an attempt to start the vehicle at a comparatively large throttle opening tends to increase an engine rotary speed until a target engine rotary speed according to the throttle opening (throttle position) is reached.
Accordingly, the engine rotary speed increases when the gear ratio position is maintained along (sticking to) a LOW position among a plurality of gear ratio positions. Then, the starting clutch comes into a partially engaged state at an engine rotary speed of, for example, 1400 rpm (clutch-OFF rotary speed), and the starting clutch comes to be engaged (connected) at an engine rotary speed of, for example, 1800 rpm (clutch-ON rotary speed).
However, there is the problem that when the starting clutch is engaged, the gear ratio position is at the LOW position, and due to the high driving force, the shock (due to engine/vehicle vibration) at the time of the connection (engagement) of the starting clutch is so great that the vehicle starting feeling is bad, e.g., shocking, bumping of the engine.
The present invention has been made to overcome such drawbacks. Accordingly, it is one of the objects of the present invention to provide a gear ratio control method, for a continuously variable transmission, by which it is possible to reduce the shock at the time of connection of a starting clutch even if the throttle opening is comparatively large at the time of starting, and it is therefore, possible to obtain a smooth vehicle starting feeling.